Typically, large loudspeakers are used to reproduce low frequencies. Many small loudspeaker systems, which for technical reasons are not suitable for emitting strong bass signals, use so-called virtual bass systems. Therein, low frequencies are replaced by their harmonics. Due to a psycho-acoustic phenomenon, the loss of the fundamental frequency is not perceived in this scenario. Known in the prior art is, for example, the audio signal processing device for improving psycho-acoustic bass perception from “Improving Perceived Bass and Reconstruction of High Frequencies for Band Limited Signals”, by R. M. Aarts, E. Larsen and D. Schobben (MPCA-2002), which is shown in FIG. 1. Therein, left and right input signals EL and ER are filtered in high-pass filters HFILL and HFILR, respectively. Furthermore, the input signals are summed and then filtered with a low-pass filter FIL1 to obtain the bass component. This bass component then is used to extract harmonics in a non-linear circuit NLD. These harmonics then pass through another filter FIL2 to obtain a suitable spectrum, and subsequently pass through an amplifier G, before they are added to the output signals of the high-pass filters HFILL and HFILR. The output signals AL and AR now no longer contain the low frequencies of which harmonics were generated. These were replaced by the so-called “synthetic” or “virtual” bass, i.e. the harmonics.
WO97/042789 shows various options, in which a level of a portion of the bass signals is detected and the generated harmonics are scaled according to this level. Thus, various non-linear circuits can be used to generate harmonics. In one version, the bass signal is split up into different frequency ranges by means of first band-pass filters BPF1A, . . . , BPF1N, as shown in FIG. 2. Subsequently, harmonics are generated for each of these frequency ranges via a non-linear circuit HGSA, . . . , HGSN, and said harmonics are scaled depending on their levels. The scaled harmonics then pass through a second band-pass filter BPF2A, . . . , BPF2N and finally are added AD to the high-pass filtered HPF input signal 10H. The output signal particularly is used for loudspeakers with high-pass characteristics, which cannot reproduce the original bass signal. Instead, its harmonics are reproduced to give the listener the impression of a bass component within the overall signal.
However, virtual bass cannot always adequately replace real bass. Significant tonal discrepancies may occur between the bass systems in certain frequency ranges.
Another technology uses level limiters, which generally are used in active loudspeakers. Therein, the level of an audio signal is limited to a certain maximum depending on frequency. However, limiting levels dynamically changes the tonality of the audio signal.
In the German patent application establishing priority, the German Patent and Trademark Office has researched the following documents: DE 697 16 216 T2 and AARTS, Ronald M. [et al]: Improving perceived bass and reconstruction of high frequencies for band limited signals. In: IEEE: Benelux Workshop on Model based Processing and Coding of Audio (MPCA-2002), Leuven, Belgium, Nov. 15, 2002, 2002, pg. 59-71.